1. Field of the Invention
The present invention relates to a rubber composition containing an ethylene-α-olefin having 3 to 20 carbon atoms-non-conjugated polyene copolymer, which is excellent in dynamic characteristics, namely its having a low tan δ, as well as excellent in mechanical characteristics, heat resistance and fatigue resistance and which is suitably used for automobile tires and rubber vibration insulator materials.
2. Description of the Related Art
The diene type rubber, such as natural rubber (NR), styrene-butadiene rubber (SBR) and butadiene rubber (BR), is known as a rubber excellent in dynamic fatigue resistance and dynamic characteristics, and it is used as a raw material rubber for automobile tires and rubber vibration insulators. These days, however, the environment wherein these rubber products are used has changed greatly, and there is requested improvement for the heat resistance and weather resistance of the rubber products.
Regarding automobile tires, treads and tire side walls particularly demand weather resistance. However, there has been hitherto no such rubber that retains superior mechanical characteristics, fatigue resistance and dynamic characteristics the conventional diene type rubber provides and in addition that possesses good weather resistance.
There have heretofore been made various studies on blend type rubber compositions comprising a diene type rubber which has excellent mechanical characteristics, dynamic fatigue resistance and dynamic characteristics and an ethylene-α-olefin having 3 to 20 carbon atoms-non-conjugated polyene copolymer, such as ethylene-propylene-non-conjugated diene copolymer rubber (EPDM), which has superior heat resistance and weather resistance. However, the levels of dynamic characteristics the ethylene-α-olefin having 3 to 20 carbon atoms-non-conjugated polyene copolymer possesses and the levels of those the diene type rubber possesses are different, so that blend type rubber compositions to exhibit uniform physical properties have not been obtained up to now. The dynamic characteristics in automobile tires are related to whether the material used does not worsen fuel consumption, and an index thereof is tan δ (loss tangent), and the lower the tan δ the better the dynamic characteristics.
Meanwhile, regarding rubber vibration insulator products for automobiles, as the temperature inside engine rooms becomes more elevated, it has become difficult for such rubber vibration insulator products based on natural rubber, i.e. conventional diene type rubber, to provide practically endurable fatigue resistance.
Accordingly, emergence of a novel rubber material is desired which has excellent heat resistance and in addition which has mechanical characteristics, dynamic characteristics and fatigue resistance equal or superior to the diene type rubber.
Generally, for improving dynamic characteristics it is necessary to achieve a higher crosslinking density. However, in the prior art, when it was tried to equalize the dynamic characteristics of an ethylene-α-olefin having 3 to 20 carbon atoms-non-conjugated polyene copolymer to those of a diene type rubber such as NR, the crosslinking density became too high, with the result that mechanical characteristics, such as tensile elongation at break, deteriorated; thus it has been impossible to make the dynamic characteristics and physical properties compatible.